1. Field of the Invention
The present invention relates to a spectroscopic ellipsometer for measuring a thickness or the like of a thin film on a surface of, for example, a glass substrate of a semiconductor wafer, a reticule/mask or a liquid crystal display (LCD).
2. Description of the Background Art
As disclosed in Japanese Unexamined Patent Publication No. 2005-308607, an ellipsometer is a device for observing a change in a polarization state when a light is reflected or transmitted by a surface of a sample and measuring optical constants (a refractive index and an extinction coefficient) of the sample or, if a thin film layer is present on the surface of the sample, measuring a layer thickness and optical constants of the thin film layer. Each specific measurement value is represented as follows using psi (Ψ) and delta (Δ) related to a ratio of a Fresnel reflection coefficient Rp of a p-polarization state and a Fresnel reflection coefficient Rs of an s-polarization state:ρ=Rp/Rs=tan(Ψ)exp(iΔ).
In the Equation, tan(Ψ) is equal to an amplitude of a ratio of a p-direction complex reflection coefficient to an s-direction complex reflection coefficient and Δ denotes a phase difference between the reflection coefficients of the p-polarization state and the s-polarization state. 
Meanwhile, a single-wavelength ellipsometer obtains a film thickness value from ellipsometric parameters such as tan (Ψ) and Δ by simple calculation. However, if a film thickness of a multilayer film is to be measured, the single-wavelength ellipsometer is required to use an extremely complicated model equation and cannot simply calculate the film thickness. Recently, therefore, development of an ellipsometer based on a method called “spectroscopic ellipsometry” for analyzing a multilayer film by performing parameter fitting and multivariate analysis while changing wavelengths is underway.
According to the spectroscopic ellipsometry method, fitting data defined by a plurality of parameters such as a film thickness, optical constants and a surface roughness of a sample is made to approximate measurement data represented by a Δ value and a Ψ value at every wavelength of a reflected light with respect to an incident light irradiated on the sample by sequentially changing the respective parameters. Further, properties of the sample are calculated based on values of the respective parameters for approximated fitting data at a time at which an error of the fitting data from the measured data is estimated to be a minimum.
Currently, an ordinary fitting calculation based on the spectroscopic ellipsometry method ends at the time at which the error between the measured data and the fitting data is estimated to be a minimum, as stated above.
Fitting will now be described. If it is assumed that N measurement data pairs are Exp (i=1, 2, . . . , N), N model calculation data pairs of the fitting data corresponding to the N measurement data pairs are Mod (i=1, 2, . . . , N), and that a standard deviation is σi on a premise that a measurement error is normally-distributed, a mean square error (x2) is represented by the following Equation:
                              χ          2                =                              {                          1              ⁢                              /                            ⁢                              (                                                      2                    ⁢                    N                                    -                  P                                )                                      }                    ⁢                                    ∑                              i                =                1                            N                        ⁢                                                            (                                                            Exp                      i                                        -                                          Mod                      i                                                        )                                2                            ⁢                              /                            ⁢                                                σ                  i                  2                                .                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
In Equation (1), P denotes a number of parameters. The fact that x2 is small is none other than a high coincidence between a measurement result and a model. Accordingly, if a comparison is made for a plurality of models, a model exhibiting a smallest x2 is considered a best model.
However, a fitting method using such a mean square error (x2) has a problem in that the method cannot be applied to an instance in which measurement data differs from fitting data in the number of pieces of data.
Moreover, the conventional spectroscopic ellipsometer has the following problems. Generally, because of differences in a measurement condition, such as an angle of incidence (AOI) of an incident light irradiated on a surface of a sample, a wavelength measurement range and the number of pieces of data, a simple comparison cannot be made between measurement data and reference data.
Furthermore, a subsequent calculation volume increases depending on a setting of an initial value of fitting data to be parameter-fit to the measurement data, resulting in consumption of time. The setting of the initial value of the fitting data greatly depends on expertise of an operator, the setting is quite a difficult operation for a beginner and the beginner is forced to perform calculation by trial and error.